The 'Lighter' Side Of The Force
Recall that Qui Gonn Jinn's body did not disapear either. It was burned at the end of the Phantom Menace. https://www.youtube.com/watch?v=O-wh4VN7ZVs
The act of selflessness the Jedi can achieve this, as the Sith are selfish and can not achieve it. As you can remember Darth Vader heard his sons prayer, and reacted as only a father could and removed the threat by killing Emperor Palpatine. By doing so he proved that love for his own son and achieved his selflessness. By the way I believe that force ghosts can show them self to anybody that has been trained as a Jedi, so I think that Obi Wan Kenobi revealed him self to Darth Vader inside his meditation chamber and told him how to this.